In 2010, McCarthy was invited by Lauren Faust, co-creator and producer for Foster's Home for Imaginary Friends, to write for Hasbro's recent revival of its My Little Pony franchise. McCarthy, on a creative hiatus at the time and "trying to figure out which direction to steer [her] writing career," accepted due to Faust's dedication to the project.[1]

In 2011, after Faust departed from the show following the season twopremiere,[2] McCarthy was promoted to co-executive producer for season three onward, during which she also served as story editor.[3]

In June 2015, Hasbro promoted McCarthy as the company's new head of storytelling for its My Little Pony and Littlest Pet Shop brands.[4]

"As I was working on "MY LITTLE PONY", I kept reforming all the villains, much to some people's dismay, who like to keep their villains mean. That just kept sort of showing up in my work. It's just my world view that no one's bad, they're just broken. And if you can figure out how to fix them, then things will be okay."